It is known to make a finished shaped workpiece from a mass of powder, a deformable mass containing powder, or a deformable mass that has been partially compacted. This is done in an apparatus having a mold comprised of a mold body defining a chamber, a mold liner in the chamber defining a cavity, and a plunger engageable in the cavity. The liner has a pressing surface exposed in the cavity, generally centered on an axis, and directed in one radial direction relative to the axis, and an opposite surface directed in an opposite radial direction, and the plunger is moved axially.
With such an apparatus the mass, typically of metal powder, is confined in the mold cavity and the plunger is driven under great force into the liner, thereby compressing and compacting the mass. In some systems there are multiple pistons, typically when the liner is a cylindrical tube there are two pistons pushed into the tube axially oppositely. Further mold elements can be advanced into the cavity to form undercuts or the like, these elements being withdrawn before the finished workpiece is demolded.
In the most common system the mold body is basically annular and the liner is a cylindrical tube centered in the mold body and formed of an extremely strong and deformation-resistant material such as a high-grade steel. The plungers are pressed axially oppositely as mentioned above into the liner to compress the mass, thereby forcing it radially outward against an inner pressing surface of the liner. Even though the liner is braced against the mold body, the enormous forces used will radially outwardly stretch the liner so that, once the plungers are withdrawn, the finished workpiece is solidly lodged in the tube and is quite difficult to remove. The mass has been deformed plastically and the liner has been deformed elastically, so these two parts are solidly fitted together. Dealing with this radial springback often puts a great deal of stress on the workpiece and on the mold liner. Even if the liner is made of hardened steel, there is still some such radial springback that retains the workpiece in the mold.
When particularly long parts are being made this problem of the workpiece getting wedged in the mold liner is particularly severe. In addition when the workpiece is long, it is necessary to use very high forces to compact the mass of powder all the way to the center, compounding the difficulties.
One solution has been to add some sort of lubricant to the powder. While a lubricant does indeed make demolding the finished part easier, it creates a finished part that is substantially softer and weaker.
WO 02/32655 of Nordell describes a powder press where the mold cavity is tapered. This makes it easier to demold the workpiece, once it has been moved a little, but to start with it is as solidly wedged in place as in a system with a cavity of nontapered cross section, and the tapered shape cannot be used in many workpieces.
German 198 30 601 of Hess describes a system where several mold parts move together to make a cruciform mold cavity in which powder is compressed. While the workpiece can be demolded relatively easily by spreading the various parts, the system has the considerable disadvantage that there is frequently leakage between the parts so that the workpiece is spoiled or needs special flash-removing operations to finish it.
Similarly, in German 195 08 952 several plungers are provided in a mold with movable undercut-forming parts. Here the structure is very complex, in particular with regard to parts that must be displaced when the powder mass is pressurized.
The system of EP 1 097 801 of Achim relates to powder pressing. Here the press actuator is a piezoactive device, but the mechanism is very complex and has the same problems with demolding the finished workpiece as the other prior-art systems described above.
In another known system called hydrostatic pressing the powder mass to be compacted is fitted in a jacket that is compressed in every direction in a body of liquid that itself is pressurized at very high pressure. Thus the mass is relatively easily separated from the jacket when the pressing job is complete, but this method does not allow for convenient mass production of standard parts In addition producing a piece to exact finish dimensions is quite difficult as they depends to a large part from the pressure employed in addition to from the shape and size of the mold.